In general, inkjet printing machines or printers include at least one printhead unit or marking engine that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink can be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the unit onto media or an imaging member. Once the ejected ink is on media, the ink droplets solidify.
The media used in both direct and offset printers can be in web form. In a web printer, a continuous supply of media, typically provided in a media roll, is entrained onto rolls or rollers that are driven by motors. The motors and rolls pull the web from the supply roll through the printer to a take-up roll. The rollers are arranged along a linear media path, and the media web moves through the printer along the media path. As the media web passes through a print zone opposite the printhead or printheads of the printer, the printheads eject ink onto the web. Along the feed path, tension bars or other rolls remove slack from the web so the web remains taut without breaking.
Existing web printing systems use a registration control method to control the timing of the ink ejections onto the web as the web passes the printheads. One known registration control method that can be used to operate the printheads is the single reflex method. In the single reflex method, the rotation of a single roller at or near a printhead is monitored by an encoder. The encoder can be a mechanical or electronic device that measures the angular velocity of the roller and generates a signal corresponding to the angular velocity of the roll. The angular velocity signal is processed by a controller executing programmed instructions for implementing the single reflex method to calculate the linear velocity of the web. The controller can adjust the linear web velocity calculation by using tension measurement signals generated by one or more load cells that measure the tension on the web near the roll. The controller implementing the single reflex method is configured with input/output circuitry, memory, programmed instructions, and other electronic components to calculate the linear web velocity and to generate the firing signals for the printheads in the marking stations.
Another existing registration control method that can be used to operate the printheads in a web printing system is the double reflex method. In the double reflex method, each encoder in a pair of encoders monitors one of two different rollers. One roller is positioned on the media path prior to the web reaching the printheads and the other roller is positioned on the media path after the media web passes the printheads. The angular velocity signals generated by the two encoders for the two rolls are processed by a controller executing programmed instructions for implementing the double reflex method to calculate the linear velocity of the web at each roller and then to interpolate the linear velocity of the web at each of the printheads. These additional calculations enable better timing of the firing signals for the printheads in the marking stations and, consequently, improved registration of the images printed by the marking stations in the printing system. A double reflex printing system is disclosed in U.S. Pat. No. 7,665,817.
While control of the rotational speed of rollers can be critical for the proper registration of images, other factors besides web transport can affect image registration. For instance, the material properties of the recording media can affect registration of images. If the continuous web slips when engaged with one or more rollers in the media path, the position of the media web with respect to the printheads can be affected and errors in images formed on the media web can occur. If the web either stretches of shrinks during imaging, misregistration of images can also occur.
Some direct marking, continuous web printers are configured to print images onto both sides of the web, also referred to as duplex printing. To enable duplex printing on a continuous web, a web transport system can be configured to print onto one side of the web and direct the web back through an inversion system that inverts, or flips, the web over so that the opposite side is facing the printhead or printheads for completing the duplex image. In some printing systems, the inversion system can be located outside the printer and the continuous web can move from the printer to the inversion system and back to the printer. To invert the web for duplex printing, known systems can include turn bars that invert the web after printing one side (e.g., simplex side), and laterally offset the web to direct the web to the entrance of the duplex web path for printing on the other side (duplex side). In some printing systems, top of form (TOF) marks, also known as fiducial marks, are used to time the application of ink and properly register images. A fiducial mark can be printed at regular intervals, for example, at the beginning of a frame during the first pass and the second pass of the recording media when being moved through the marking engine. A frame is defined to include both the length of an imaging area on the recording media taken in the process direction and a non-imaging area located between a first image area and a second image area. The non-imaging area is called an inter-panel zone or inter-document zone.
In some printing systems, the printheads can deposit the alignment marks, for instance a pattern of dashes on the print media. The dashes can be imaged by a sensor, such as an ink on web array (IOWA) sensor. The dashes being imaged by the IOWA sensor can provide location information of the frames in the process direction. Because location of the nozzle or nozzles which produce each dash is known, the position of every print head can also be determined from the alignment marks.
A sensor detects the presence of the fiducial marks on the first pass and the second pass through the marking engine and provides signals to a controller configured to control the application of ink to the continuous web. In printers incorporating a duplex web path with an inversion system, the continuous web includes printed images on one side of the web as the web moves along a transport path from the printer to the inversion system and back to the printer. During this movement, the length of the web can change. Some sources of the change in length can be moisture loss in the paper due to the elevated temperature of the paper. To print properly aligned duplex images, the changes in the length of the paper path should be considered.